Moralistic and Naturalistic Fallacies

Is refers to what is; that means, what actually happens or exists. The observable, natural world, and facts that describe that world.

Examples:

- The existence of the sun (the sun does exist)

- Human mortality (humans are mortal)

Ought refers to what ought to be; that means, what this observable world should be like, or what should/shouldn't be in it. 

Examples:

- Gender equality (men and women should be treated equally)

- Food security (people should have enough to eat)

The is-ought principle tells us not to derive moral judgements straight from facts. For instance, the fact that the sun exists doesn't mean it should exist (but that doesn't mean it shouldn't, either). Similarly, we cannot derive facts straight from moral judgements - just because we believe men and women should be equal under the law, does not mean they are literally equal, as in, the exact same. When moving from is to ought, you need to state an ethical principle (admitting that it is your values, not the facts directly, that led you to a belief); likewise, when moving from ought to is, only parse the factual parts of your argument, not the ones rooted in opinion.

You get a fallacy when you ignore this principle: naturalistic fallacy when you derive 'ought' from 'is' (e.g. a woman is able to give birth, therefore women should be homemakers), or moralistic fallacy when you derive 'is' from 'ought' (e.g. the sun should not exist, therefore it's not actually real, just constructed).